Richard Barnard braves the mess and overcomes key challenges for the satisfaction of a clean fuel tank, free from sediment and sludge

Cleaning a diesel tank: a practical guide

There can’t be many of us who sail boats with engines who haven’t experienced the dying clunks of our engine as it grinds to a halt.

The most common reason is a blockage in the fuel system, caused by sediment in the fuel, or the dreaded diesel bug clogging the fuel line.

You can minimise these risks by using a filter funnel and adding anti-bug chemicals whenever you fill up your tank.

I’ve always taken these precautions and managed to avoid problems with fuel until this season, when a particularly rough sea stirred up trouble in my tank.

After I got home under sail, I checked the primary fuel filter and found it jammed solid with a brown, jelly-like substance, with some grit within it.

A disconnected diesel fuel tank on a boat

The tank is ready to disconnect and move. Credit: Richard Barnard

The tank drain hole is located in a front corner of the tank, directly below the bottom of the fuel feed pipe.

Nothing came out of this drain, but I was able to suck it clear using a hand pump, which pulled out more of the jellied sediment.

There was clearly a major problem that changing the fuel filter was never going to resolve and I’d have to clean out the tank.

With the drain hole now clear I drained off the 40lt of diesel sitting in the 250lt tank, grateful that I hadn’t recently filled it!

The top of a diesel fuel tank on a boat

Tank top beneath the saloon seat. Credit: Richard Barnard

By removing the fuel gauge unit on the tank top, I created a 2in diameter hole through which I could insert a small torch and squint.

I could also insert a length of bamboo and feel the bottom of the tank. I could see, and feel, sludge about half an inch deep.

Three key challenges when cleaning a diesel tank

At 250lt (65gal) the tank was large and difficult to remove from the boat. It was sited beneath a saloon seat with limited access to its top.

There were no access hatches to allow inspection of the interior or cleaning.

Given the absence of hatches it’s likely the tank had never been properly cleaned since it was fitted, and that the sediment was just an accumulation of decades of dirt in the fuel and some bacterial activity.

A freed diesel fuel tank on a boat with holes marked for drilling

Tank freed and holes marked. Credit: Richard Barnard

Whatever the reasons, I would have to fit three hatches in the tank top, above the three baffled sections, to allow access for cleaning.

To do that I’d have to release the tank and slide it into the centre of what was fortunately a large engine room bilge area.

I calculated that if I could find hatches of about 5in (125mm) in diameter, I’d be able to fit one above each baffled section of the tank, which would allow me to get my arm in for cleaning.

A man cutting holes in a diesel tank on a boat

Cutting the hatch access holes. Credit: Richard Barnard

I should also be able to cut the holes without having to remove the tank completely.

I searched for tank access hatches on the internet, finding the perfect one: an FLB tank inspection hatch, produced by Wema UK.

This is made up of two strong, plastic, circular plates, joined by a central bolt.

holes cut into a hatch on a boat

Hatch holes cut. Credit: Richard Barnard

By cunning shaping of the lower plate, it is possible to slide the unit into and over the cut hole and tighten the bolt, which pulls the plates together, sealing the top plate with a substantial O-ring.

Before unbolting the tank I marked out the places on the top, between the timber sole bearers, where the hatches should be fitted.

Continues below…

Disconnecting and releasing the tank was easy, but sliding it out and securing it in place for working on required a bit of care to ensure I didn’t create any problems with the prop shaft or wiring.

My friendly local metalworker used his skill and stainless steel jigsaw cutting blades to cut the three circular holes and I was then able to have a better look within.

A dirty diesel fuel tank on a boat

The diesel feed pipe before cleaning. Credit: Richard Barnard

The foul brown sludge lay all over the bottom of the tank, clearly visible through the holes and in the photos taken by my phone as I carefully lowered it into the tank.

To remove this sludge I used a cut milk carton as a scoop, pouring the sludge into a plastic pint glass, which I then periodically lifted out to transfer the sludge into a jerry can.


The sludge being scooped out. Credit: Richard Barnard

It was messy, but it wasn’t long before I could use a big sponge to squeegee the tank, rinsing the sponge in buckets of fresh water.

When it was as clean as I could make it this way, I washed the tank interior with a degreasing agent in water, throwing the liquid all around the inside of the tank and using a long-handled brush to get into all the corners.

A man wearing a blue jumper drying out a fuel tank

Blowing hot air in to dry the interior of the tank. Credit: Richard Barnard

By luck, the tank had orientated itself at a slight angle when I pulled it out, and the drain hole was at its lowest point.

So all this washing out liquid drained away into the sump, from where I could scoop it out later for safe disposal.

access hatches on a marine diesel fuel tank on a boat

Circular access hatches fitted for future cleaning. Credit: Richard Barnard

The tank now looked really clean and I used a hot air gun to start drying out the interior, leaving the tank for a couple of days to completely dry out.

I then fitted the hatches and slid the tank back into place, re-bolting it to its framework and re-connecting it to the fuel filter and engine.

The top of a marine diesel tank

Tank back in place with its new access hatches. Credit: Richard Barnard

The last job was to replace the tank fuel gauge and pour in fresh diesel. With new fuel filters in place, the engine fired up and ran perfectly.

It was so reassuring to know that there was now no risk of future engine failure due to fuel problems, however much the boat might rock around.

The job was very satisfying to do and, if and when I get another boat, I’ll make sure I can inspect inside its fuel tank and ensure it’s clean and free from sediment and sludge.